Foundry iron, used for casting and steel making, is produced in the iron industry in a number of different processes. The process used is typically dependent on the feed material and the intended use of the foundry iron.
One process of producing foundry iron utilizes a standard cupola-type furnace. A variety of iron sources such as scrap iron, scrap steel and pig iron are fed into the vertical shaft of the furnace fueled by combustion of coke by a blast of air. The charge added to the furnace generally contains a number of additives such as ferrosilicon to increase the silicon content of the iron and slag forming materials such as limestone to remove impurities such as sulfur. The iron produced by this process typically contains about 1 percent to 3 percent silicon and about 2 percent to 4 percent carbon.
The cupola-type furnace disadvantageously is a net silicon oxidizer with the result that as much as 30 percent of the available silicon charged is lost by oxidation and discharged in the slag. Typically, only about 70 percent of the available silicon charged reports to the iron. Silicon is an essential element of foundry iron and is typically added in the form of ferrosilicon since such a form of silicon is readily combinable with the iron. Ferrosilicon is an expensive source of silicon such that silicon losses through oxidation can significantly increase production costs.
The cupola-type furnace is often desirable since it can be energy efficient and requires a relatively low capital investment. A cupola furnace is also easily scaled up for large production from a single unit and can be operated as a continuous charging and tapping process. Carbon is easily combined with the iron and is picked up naturally in the cupola as the melted iron and steel droplets pass over the hot coke and dissolve the carbon.
The feasibility of producing foundry iron is dependent in part on the efficiency of the process used and cost of the charging materials. The cost of scrap iron and scrap steel depends on several factors including the iron content, amounts of desirable and undesirable alloy constituents present, and the particle size. The cost of very fine or light scrap iron and steel, such as borings or turnings, is typically much less than heavier scrap such that it is desirable to use light scrap whenever practical. The use of light scrap in a cupola requires agglomeration or briquetting since the high volume of gases exiting the cupola otherwise carries an unacceptably large percentage of the charge from the furnace. Very fine or light iron scrap will be collected in the baghouse or scrubber resulting in a low recovery of iron and thus increased operating cost.
Foundry iron is also produced conventionally and commercially with the electric induction furnace. In the electric induction furnace the charge, which can be iron scrap, steel scrap and pig iron, is introduced into the furnace, melted; and, then additives, including silicon, carbon, and a slag forming material to cover the iron are introduced. The iron charge is heated by eddy currents resulting from electromagnetic induction from the alternating electric current flowing in the coil surrounding the charge. Silicon is typically added as ferrosilicon, and carbon is added in the form of a low sulfur content graphite material. The resulting iron generally has a silicon content of about 1-3 percent and a carbon content of about 2-4 percent.
The electric induction furnace disadvantageously is limited to a batch process where individual units are typically capable of producing less than 20 tons of iron per hour. In addition, the electric energy is typically applied only about 80% of the time, resulting in inefficiency. Other disadvantages include the moderate to high refractory costs, high capital investment, high labor costs, high cost of ferrosilicon and carburizing additives, and limited scale up capability of the induction furnace.
Although not usually economical, foundry iron has been produced commercially in standard-type electric arc furnaces (EAF). The EAF typically consists of a refractory lined vessel or shell with a removable refractory roof through which three electrodes in a three phase AC furnace, or one electrode in a DC furnace protrude into the space above the charge material and bath contained within the furnace shell. For DC furnaces, the return electrode is typically built into the bottom of the furnace shell.
The operation of the electric arc furnace typically consists of charging the furnace by swinging the roof aside and emptying one or more charge buckets containing iron or steel scrap and other materials into the shell, closing the roof, and then lowering the electrodes until contact is made with the charge and arcing and melting of the charge occurs. After melting, a slag layer is usually established for refining purposes, and additions of ferrosilicon and carbon are made until the foundry iron composition reaches the desired target. The furnace is then tapped, any needed refractory repairs made, and the cycle repeated.
In recent years, the EAF has not been used extensively for production of foundry iron alloys because of the relatively high production cost. The EAF is not economical for the production of foundry irons because of the high cost of ferrosilicon and carbon additions required, and because it is a batch process. Presently, the use of the EAF has been mostly limited by economics to the production of special alloy foundry irons, and steels, which may not be readily or economically produced in either cupolas or in induction furnaces.
Another process of producing foundry iron is by smelting iron ore in a submerged arc electric furnace. Submerged arc furnaces have an advantage of directly smelting the ores using the heat of the electric arc along with simultaneous carbothermic chemical reduction of metal oxides by the carbonaceous reducing agents, such as coke and coal. The electrodes are immersed in the charge and slag layer which forms above the molten iron. That arrangement permits efficient heat transfer between the arc and charge materials. However, the nature of the heating in the submerged arc furnace requires that the electrical conductivity of the charge be controlled to permit the simultaneous immersion of the electrodes deep into the charge while avoiding excessive currents in the electrodes, which excessive currents could cause the electrodes to overheat.
Iron ore has low electrical conductivity making it amenable to smelting in a submerged arc furnace. The prior production of foundry iron in submerged arc furnaces generally uses iron ore in the form of fines, lumps or pellets as the primary source of iron. One example of the use of a submerged arc furnace to smelt iron ore is disclosed in U.S. Pat. No. 4,613,363 to Weinert. A disadvantage of the conventional iron producing processes using a submerged arc furnace is that the carbothermic reduction of ores to produce iron requires large amounts of electric energy, thereby increasing the production costs. Alternatively, the more widely utilized processes of producing foundry iron (cupola and induction furnaces) require comparatively expensive starting materials, such as heavy iron or steel scrap; and prior-reduced silicon sources such as silicon carbide or ferrosilicon, which are relatively expensive sources of silicon. All of these characteristics have limited these prior processes for producing foundry iron.
More recently, submerged arc furnaces have been used to melt scrap metal as disclosed in U.S. Pat. No. 5,555,259 to Feuerstache. The furnace is formed with a center pipe surrounding the cathode which prevents the charge from contacting the side of the cathode. The cathode extends beyond the pipe to form an arc between the cathode and an anode and melt the charge. The lower end of the pipe is tapered for feeding the scrap to the cathode. The pipe surrounding the cathode enables the cathode to be positioned deep in the charge bed. This construction, however, has several disadvantages including, for example, the complex and expensive water cooled components and complex electrical connection for the electrodes. In addition, the water cooled components within the shell of the electric arc furnace near the electric arc create potential safety concerns.
Accordingly, the iron foundry industry has a continuing need for an economical and efficient process for producing foundry iron.